A lumped constant non-reciprocal circuit device has long been used as an isolator or circulator in a mobile communication device or mobile communication terminal because it requires less space. An isolator is placed between the power amplifier and antenna in the transmitter of a mobile communication device in order to, for example, prevent unwanted signals from reversely entering the power amplifier from the antenna for a desired frequency band or to stabilize impedance on the load side of the power amplifier; a circulator is used in a transmission/reception branch circuit etc.
FIG. 15 is a transparent perspective view illustrating the internal structure of a conventional lumped constant circulator (referred to below simply as circulator 100). FIG. 16 is a circuit diagram illustrating the equivalent circuit of the circulator in FIG. 15. In the equivalent circuit in FIG. 16, a ferrite plate F1 is not shown.
As shown in FIG. 15, in conventional circulator 100, three center conductors L1, L2, and L3 (each of which has two linear conductors having both ends grounded) mutually insulated and superimposed one another so as to intersect at an angle of 120 degrees are placed between a ferrite plate F1 and a ferrite plate F2 (not shown) of the same shape as ferrite plate F1, and permanent magnets (not shown) for magnetizing ferrite plates F1 and F2 are disposed facing each other so as to sandwich ferrite plate F1 and F2 therebetween.
One end of each of center conductors L1, L2, and L3 projects externally from the rims of ferrite plates F1 and F2 and the projection is connected to a signal input/output port (not shown) and one end of each of matching dielectric board pieces (matching capacitors) C1, C2, and C3. The other end of each of center conductors L1, L2, and L3 and the other end of each of matching dielectric board pieces (matching capacitors) C1, C2, and C3 are grounded electrically. Center conductors L1, L2, and L3 have inductance. When a lumped constant circuit element is used as an isolator, the input/output port of center conductor L3 is connected to one end of a terminator and the other end is grounded electrically to absorb reflected signals.
In a structure as described above, if the matching conditions by matching capacitors, the inductances of the center conductors, and the materials of ferrite plates F1 and F2 are optimized, circulator 100 shows irreversibility in a certain frequency range. That is, circulator 100 has high attenuation characteristics (isolation) for a signal that is input to the input/output port connected to one end of the center conductor L1 and output from the input/output port connected to one end of the center conductor L2, a signal that is input to the input/output port connected to one end of the center conductor L2 and output from the input/output port connected to one end of the center conductor L3, and a signal that is input to the input/output port connected to one end of the center conductor L3 and output from the input/output port connected to one end of center conductor L1; circulator 100 has low attenuation characteristics (or opposite characteristics) for signals that are transmitted in the directions opposite to those. If a terminator R1 is connected to the input/output port of the center conductor L3, the non-reciprocal circuit device functions as an isolator, in the corresponding frequency band, which has high attenuation characteristics for a signal that is input to the input/output port connected to one end of the center conductor L1 and output from the input/output port connected to one end of center conductor L2 and has low attenuation characteristics (or opposite characteristics) for signals that are transmitted in the direction opposite to that.
However, the frequency (operating frequency) bandwidth in which a non-reciprocal circuit device such as a conventional isolator or circulator shows irreversibility is generally narrow. (For example, the frequency bandwidth that gives attenuation with an irreversibility of 20 dB at a center frequency of 2 GHz is several tens of hertz.).
Non-patent literature 1 discloses technology for widening the bandwidth of the operating frequency of an isolator. This known technology achieves a bandwidth ratio of 7.7% at a center frequency of 924 MHz by adding an inductor or capacitor to the input end of an isolator. Non-patent literature 2 discloses an example of increasing the fractional bandwidth to 30 to 60% by adding an inductor or capacitor between a center conductor and the ground. Patent literature 1 discloses technology for widening the bandwidth without increasing insertion loss by providing a capacitor between a ground conductor connected to one end of each of three center conductors and the ground. In the above methods of widening the bandwidth, however, there are limits to the extent to which the bandwidth of operating frequency can be widened due to insertion loss or degradation in isolation characteristics, so it is difficult to use these methods for application in which two frequency bands significantly apart (for example, more than one octave band apart) must be covered.
Patent literature 2 discloses a non-reciprocal circuit device that changes the operating frequency with an RF switch for disconnecting or connecting a capacitor disposed on the input/output port of each center conductor to change the resonance frequency of a resonant circuit. In this structure, however, the operating frequency is toggled with the switch, so concurrent use in a plurality of frequency bands is impossible, thereby disabling its usage in an environment in which a plurality of applications for different frequency bands are implemented concurrently. Patent literature 3 discloses a non-reciprocal circuit device that changes operating frequency bands by changing the reactance of a variable capacitor disposed on mutual connection ends of the three center conductors. Since reactance needs to be changed in this structure, however, it is not applicable to an environment in which a plurality of applications for different frequency bands are implemented concurrently as in the structure in patent literature 2.
Patent literature 4 discloses a structure in which two isolators are placed in series with two ferrite plates for dual-band support using an installation area of the size equivalent to that for a single band isolator. However, application to portable terminals is difficult because the height is increased in this structure.    Non-patent literature 1: Hideto Horiguchi, Youichi Takahashi, Shigeru Takeda, “Out-band Attenuation Enhancement and Bandwidth Enlargement in a Small Isolator”, Hitachi metals technical review, vol. 17, pp. 57-62, 2001.    Non-patent literature 2: H. Katoh, “Temperature-Stabilized 1.7-GHz Broad-Band Lumped-Element Circulator”, IEEE Trans. MTTS Vol. MTT-23, No. 8 August 1975.    Patent literature 1: Japanese Patent Application Laid-Open No. 11-234003    Patent literature 2: Japanese Patent Application Laid-Open No. 9-93003    Patent literature 3: U.S. Pat. No. 3,605,040    Patent literature 4: Japanese Patent Application Laid-Open No. 2001-119210